The invention relates to a measuring system for measuring an imaging quality of an EUV lens, an arrangement comprising an EUV lens for an inspection apparatus and a measuring system of the aforementioned type, an inspection apparatus for inspecting a surface of a substrate for microlithography, and a method for measuring an imaging quality of an EUV lens.
The measurement of wavefront aberrations for EUV lenses at the operating wavelength thereof is a crucial factor for qualification and ensuring the performance thereof. Existing solutions are optimized for systems having a reduction factor of 4:1. The need for EUV inspection optical units in the semiconductor industry pushes optical designs having a high magnification scale. On account of this magnification, interferometric measuring methods for EUV lithography optical units can achieve the required measurement accuracy only with difficulty.
Therefore, there is a need for non-interferometric measuring techniques for magnifying EUV lenses such as are required for inspection apparatuses for mask or wafer inspection which achieve the required measurement accuracy.
In order to obtain highly accurate measurement results, a wavefront source having high spatial resolution is required in order that the pupil is filled by diffraction. At wavelengths of greater than 100 nm, materials such as metals, for example, exhibit a distinct optical effect in the case of layer thicknesses below the wavelength. Only materials having small differences in the complex refractive index can be used in EUV applications. Therefore, diffractive structures have a non-negligible vertical extent which has to be taken into account in the description of the measurement. A non-optimized wavefront source, on account of its topography, can introduce artificial wavefront aberrations into the metrology system.